Genetic analyses in Drosophila have been extremely useful in dissecting the complexity of developmental pathways and identifying interacting loci. However, understanding the precise nature of the processes that underlie genetic interactions requires a knowledge of the biochemical properties of the protein products of the genes in question.
Null mutations in any one of the zygotic neurogenic loci--Notch (N), Delta (Dl), mastermind (mam), Enhancer of Split (E(spl)), neuralized (neu), and big brain (bib)--result in hypertrophy of the nervous system at the expense of ventral and lateral epidermal structures. This effect is due to the misrouting of epidermal precursor cells into a neuronal pathway, and implies that neurogenic gene function is necessary to divert cells within the neurogenic region from a neuronal fate to an epithelial fate. Studies that assessed the effects of laser ablation of specific embryonic neuroblasts in grasshoppers (Doe and Goodman 1985, Dev. Biol. 111, 206-219) have shown that cellular interactions between neuroblasts and the surrounding accessory cells serve to inhibit these accessory cells from adopting a neuroblast fate. Together, these genetic and developmental observations have led to the hypothesis that the protein products of the neurogenic loci function as components of a cellular interaction mechanism necessary for proper epidermal development (Artavanis-Tsakonas, 1988, Trends Genet. 4, 95-100).
Sequence analyses (Wharton et al., 1985, Cell 43, 567-581; Kidd et al., 1986, Mol. Cell. Biol. 6, 3094-3108; Vassin et al., 1987, EMBO J. 6, 3431-3440; Kopczynski et al., 1988, Genes Dev. 2, 1723-1735) have shown that two of the neurogenic loci, Notch and Delta, appear to encode transmembrane proteins that span the membrane a single time and suggest that these proteins could function in cell-cell interactions. The Notch gene encodes a .about.300 kd protein (we use "Notch" to denote this protein) with a large N-terminal extracellular domain that includes 36 epidermal growth factor (EGF)-like tandem repeats followed by three other cysteine-rich repeats, designated Notch/lin-12 repeats (Wharton et al., 1985, Cell 43, 567-581; Kidd et al., 1986, Mol. Cell Biol. 6, 3094-3108; Yochem et al., 1988, Nature 335, 547-550). Delta encodes a .about.100 kd protein (we use "Delta" to denote DLZM, the protein product of the predominant zygotic and maternal transcripts; Kopczynski et al., 1988, Genes Dev. 2, 1723-1735) that has nine EGF-like repeats within its extracellular domain (Vassin et al., 1987, EMBO J. 6, 3431-3440; Kopczynski et al., 1988, Genes Dev. 2, 1723-1735). Although little is known about the functional significance of these repeats, the EGF-like motif has been found in a variety of proteins that interact with other proteins, including those involved in the blood clotting cascade (Furie and Furie, 1988, Cell 53, 505-518). In particular, this motif has been found in extracellular proteins that bind to other proteins, such as the blood clotting factors IX and X (Rees et al., 1988, EMBO J. 7, 2053-2061; Furie and Furie, 1988, Cell 53, 505-518), in other Drosophila genes (Knust et al., 1987, EMBO J. 761-766; Rothberg et al., 1988, Cell 55, 1047-1059), and in some cell-surface receptor proteins, such as thrombomodulin (Suzuki et al., 1987, EMBO J. 6, 1891-1897) and LDL receptor (Sudhof et al., 1985, Science 228, 815-822). A protein binding site has been mapped to the EGF repeat domain in at least two such proteins, thrombomodulin and urokinase (Kurosawa et al., 1988, J. Biol. Chem 263, 5993-5996; Appella et al., 1987, J. Biol. Chem. 262, 4437-4440).
An intriguing array of interactions between Notch and Delta mutations has been described (Vassin, et al., 1985, J. Neurogenet. 2, 291-308; Shepard et al., 1989, Genetics 122, 429-438; Xu et al., 1990, Genes Dev., 4, 464-475). A number of genetic studies (summarized in Alton et al., 1989, Dev. Genet. 10, 261-272) has indicated that the gene dosages of Notch and Delta in relation to one another are crucial for normal development. A 50% reduction in the dose of Delta in a wild-type Notch background causes a broadening of the wing veins creating a "delta" at the base (Lindsley and Grell, 1968, Publication Number 627, Washington, D.C., Carnegie Institute of Washington). A similar phenotype is caused by a 50% increase in the dose of Notch in a wild-type Delta background (a "Confluens" phenotype; Welshons, 1965, Science 150, 1122-1129). This Delta phenotype is partially suppressed by a reduction in the Notch dosage. Recent work in our laboratories has shown that lethal interactions between alleles that correlate with alterations in the EGF-like repeats in Notch can be rescued by reducing the dose of Delta (Xu et al., 1990, Genes Dev. 4, 464-475). Xu et al. (1990, Genes Dev. 4, 464-475) found that null mutations at either Delta or mam suppress lethal interactions between heterozygous combinations of certain Notch alleles, known as the Abruptex (Ax) mutations. Ax alleles are associated with missense mutuations within the EGF-like repeats of the Notch extracellular domain (Kelley et al., 1987, Cell 51, 539-548; Hartley et al., 1987, EMBO J. 6, 3407-3417).
Notch is expressed on axonal processes during the outgrowth of embryonic neurons (Johansen et al., 1989, J. Cell Biol. 109, 2427-2440; Kidd et al., 1989, Genes Dev. 3, 1113-1129).
A recent study has shown that certain Ax alleles of Notch can severely alter axon pathfinding during sensory neural outgrowth in the imaginal discs, although it is not yet known whether aberrant Notch expression in the axon itself or the epithelium along which it grows is responsible for this defect (Palka et el., 1990, Development 109, 167-175).